This invention relates to autofocus devices and methods for use in lithography.
Currently, at the sites which manufacture semiconductor devices, circuit devices are mass-produced by using reduction projection exposure devices, using the i line from mercury lamps that has a wavelength of 365 nm as an illumination light. At the same time, the introduction of exposure devices of the next generation, having minimum line widths of 0.25 μm, has begun.
A scanning exposure apparatus based on the step-and-scan method is being developed. This apparatus uses an ultraviolet pulse laser beam with a wavelength of 248 nm from a KrF excimer laser source or an ultraviolet pulse laser beam with a wavelength of 193 nm from an ArF excimer laser source as an illumination light. A scanning exposure apparatus then linearly scans a mask or a reticle (to be generically referred to as a “reticle” hereinafter) on which a circuit pattern is drawn and a wafer serving as a photosensitive substrate, relatively to the projection field of a reduction projection optical system. This allows the transfer of the entire pattern within a shot area on the wafer by repeating the inter-shot stepping operation and the scanning exposure operation.
It seems apparent that the integration degree of semiconductor devices will further increase, requiring increased resolution of the scanning exposure apparatus. In order to increase this resolution, it is extremely effective to decrease the wavelength λ For this reason, an EUV exposure apparatus using light in the soft X-ray region of 5 to 15 nm in wavelength (“EUV (Extreme Ultraviolet) light”) as the exposure light has been developed.
In semiconductor lithography systems in use today, automatic focusing and leveling (AF/AL) is typically accomplished by passing a low angle of incidence (“glancing angle”) optical beam onto the surface of a silicon wafer and detecting its position after subsequent reflection from the wafer surface. The wafer height is determined by optical and electrical processing of the reflected light beam. This beam passes under the last element of the projection lens. The source and receiver optics are typically mounted to a stable part of the system, close to the projection optics mounting position. Signals from the AF/AL unit go to the wafer stage controller which adjusts the wafer height and leveling appropriately, so the wafer lies in the focal plane of the projection optics.
In an EUV exposure apparatus, a reflection type reticle is required, due to the high absorption of EUV light by all materials. This reflection type reticle is obliquely irradiated with illumination light and light reflected by the reticle surface is projected on a wafer through a reflective projection optical system. As a consequence, a pattern, which is irradiated with the illumination light in an illumination area on the reticle, is transferred onto the wafer.
For this reason, the projection optical system becomes non-telecentric on the reticle side. As a consequence, the displacement of the reticle along the optical axis appears on the wafer as a magnification change in the longitudinal direction of a ring-shaped exposure area (an area on the wafer which corresponds to the above ring-shaped illumination area on the reticle), and as a positional change in the transversal direction.
Non-telecentric projection optical systems are very sensitive to reticle displacement. When, for example, the reticle is displaced by 1 μm in the vertical direction (Z direction), while illuminated by radiation incident at an oblique angle of 100 mrad, an image shift of 25 nm (assuming a 4× reduction optical system), occurs on the wafer. The allowable overlay error in the semiconductor process of a device rule of 100 nm L/S is said to be 30 nm or less. Therefore, an overlay error as large as 25 nm caused by a displacement of a reticle in the Z direction alone poses a serious problem. This is because overlay errors of about 10 nm can be caused by other factors, e.g., alignment accuracy of a reticle and wafer, wafer stage alignment accuracy including stepping accuracy, or the distortion of the projection optical system.
Conventional glancing angle AF devices are large and may not have the accuracy required for future applications. For application to EUV reticle height sensing, the glancing angle of such AF devices may cause interference between the AF beams and proximity illumination blinds or other structures. Also, their accuracy may be inadequate for the <50 nm height tolerance of the EUV reticle.
Conventional glancing angle AF devices also pose problems for immersion lithography systems. In immersion lithography, a liquid such as water fills the space between the last surface of the projection lens and the wafer. At the edge of the water, typically at the edge of the lens or supported structure near the edge of the lens, the liquid-air boundary may not be well defined and may be changing rapidly. Temperature gradients in the water also cause problems for the AF. Transmitting an AF/AL beam through this interface using prior art AF systems causes substantial disruption and subsequent loss of signal, and hence performance.
It is therefore a general object of this invention to provide improved AF systems and methods for lithography.